KVM, AMD & Interrupts

[Matthew Hall <matt-Etm38r6YSMlqcVXhcSD7Ah2eb7JE58TQ@public.gmane.org>]

Hi Avi,

I spoke to you a while ago on #kvm on freenode - I had a problem with 
ide ops on an amd svm host with kvm. The issues regarded the inability 
to access ida devices (hda/hdc) under qemu & kvm on my host - if this 
triggers your memory...

The last point I got to was that it was kvm and amd specific. Id est, 
the same image run under an intel vmx host with the *exact* same image, 
kvm version and command options (svn 'trunk' from the $date).
The issue seemed to be AMD and kvm specific (running with '-no-kvm' made 
everything work ok, but slower, as you'd expect).

I got word of a possible workaround, from one of your colleagues whilst 
you've been away, to use the boot options 'noapic nolapic' on the host 
o/s. Using these options solved my hd* op problems and I can boot ok now.
It seems disabling the local apic was the fundamental part (iirc), as 
without host support the guest would always start the dummy apic; so 
running the guest with any 'special' options (ie. '-no-apic') should not 
be required.

I'm think this problem is specific to my particular cpu/motherboard 
combination aswell.
I also have box with a recent intel cpu (with the vmx extention) which 
doesn't display this behaviour; and from talking with you it's not 
purely amd specific (I believe your tests on amd cpu worked).
Is it possible a slowdown in the interrupt controller between the guest 
and host is a contributing factor?
Iirc, with a 'dummy' apic, I experience a ~5% slowdown on disk and 
network operations, which isn't so bad. I've yet to think about smp usage.

Thought you might appreciate this info should you feel it deserve a wiki 
entry. Point me in the direction of the appropriate page should you feel 
I can put this info in myself.


Best regards,
Matt

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